net: mvmdio: define module alias for platform device

The mvmdio driver can be instantiated using device tree or as a classic
platform device.  In order to load the driver automatically by udev in
the latter case, the driver needs to define a module alias for the
platform device.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>